Protesters organized by Occupy Wall Street and Occupy Together yell slogans and hold signs at Courthouse Square in Dayton, Ohio, Wednesday. Several large unions have come out in support of Occupy Wall Street.

Al BehrmanAP

Occupy Wall Street is getting a shot in the arm, as some of America's largest unions have announced that they're now supporting the movement. The gain in momentum comes as off-shoots of the original Manhattan group plan marches and protests around the nation.

Update at 7:35 p.m. ET: Thousands of people took part in today's march in Manhattan, with various estimates ranging from 2,000 and upwards. ABC News live-blogged the event. Our original post continues:

The AP notes the group's fast growth into a movement:

The protests began two and a half weeks ago with a few dozen demonstrators who tried to pitch tents in front of the New York Stock Exchange. Since then, hundreds have set up camp in a park nearby and have become increasingly organized, lining up medical aid and legal help and printing their own newspaper.

And in a sign of technical sophistication, the group also has its own Bitcoin address, where it's accepting donations. Wednesday, protests inspired by Occupy Wall Street led to protests in cities from Dayton, Ohio, to San Francisco, Calif.

And in New York City, the group is marching on Wall Street once again today — a notice on its website says that at 4:30 p.m. ET, "the 99% will march in solidarity with #occupywallstreet from Foley Square to the Financial District, where their pensions have disappeared to, where their health has disappeared to."

The group has attracted some mockery, largely for its members' proclivity for dressing up like zombies. But a new Rasmussen poll finds that the group enjoys a higher approval rating (33 percent) than does Congress (14 percent).

Perhaps sensing a groundswell of opinion, several key Democrats have endorsed the group, including former Sen. Russ Feingold and Rep. John Larson, who called it a sign of a coming "American autumn" — a reference to the Arab Spring protests that have reshaped parts of the Middle East.

And like their Arab counterparts, the key members of Occupy Wall Street seem to be young people frustrated by a lack of opportunity, and angered by the disparity between a suffering middle class and the wealthiest citizens — as personified, here in America, by Wall Street bankers.

Just as the Tea Party movement has refused to join the Republican rank and file, the political establishment may also have a hard time assimilating Occupy Wall Street's grassroots energy. Or, as The Washington Post's James Downie writes, "at least not without Democrats renouncing the influence Wall Street holds on them, as well."

From the Occupy Wall Street site, here's a partial list of the union groups that announced their support Wednesday: